It is known from U.S. Pat. No. 3,437,622 that thermoset reaction products of polyols with aromatic polyisocyanates compounded with tackifying resins and plasticizers can be used for high-temperature-resistant masking tapes.
European patent application publication No. 0 709 416 describes pressure sensitive adhesives and laminating adhesives which contain thermoset polyurethanes. In one embodiment the polyurethane is prepared from a polyisocyanate having a functionality of from 2.2 to 10 and a mixture of a hydrogenated polydiene mono-ol and a hydrogenated polydiene diol. According to another embodiment the polyurethane is prepared from a hydrogenated polydiene diol and a polyisocyanate having a functionality of from 2.2 to 10, wherein the NCO:OH molar ratio is between 0.3:1 and 0.7:1.
U.S. Pat. No. 4,820,368 describes a thermally stable reactive hot melt urethane adhesive composition having a thermoplastic polymer, a compatible curing urethane polyalkylene polyol pre-polymer, and a tackifying agent. The polyalkylene polyol is preferably a hydrogenated polybutadiene diol, either having a 1,2 vinyl content before hydrogenation of 0 to 20% by weight or 80 to 100% by weight. The NCO:OH ratio in the urethane pre-polymer is well above 2 in all examples.
The structural adhesive of the present invention typically does not contain a thermoplastic polymer. A different hydrogenated polybutadiene polyol is used and the NCO:OH ratio is lower, thus leading to a more flexible structural adhesive which is easier to process.
EP-A-0 504 436 describes a two-part polyurethane adhesive. The polyurethane preferably has a NCO:OH ratio of 2 to 10. The two component system consists of a main component comprising a hydroxyl-terminated saturated polyester resin or a polyester urethane rubber and a curing agent comprising a urethane pre-polymer having an active isocyanate group which is prepared by reacting a hydrogenated hydroxyl terminated polybutadiene with an excess amount of a polyisocyanate. The hydroxyl-terminated polybutadiene can have a 1,2 vinyl content of 0-100%. In the examples, a hydroxyl-terminated polybutadiene having a 1,2 vinyl content prior to hydrogenation of about 20% by weight is used.
EP-A-0 522 658 describes functionalised block copolymers cured with isocyanates, for example for use in coatings. The functionalised block copolymers are typically functionalised, hydrogenated styrene-butadienestyrene block copolymers. In comparative example 4, a hydroxy functional hydrogenated polybutadiene, having a 1,2 vinyl content prior to hydrogenation of 84% by weight, is reacted with triisocyanates at a NCO:OH ratio of 1.25 and 1.5 and formed into a coating.
However, there is a need for fully crosslinked structural adhesives which can be used at high temperatures. Further, there is a need for structural adhesives having better cohesion properties and being less brittle whilst exhibiting enhanced stability. In particular, there is a need for flexible structural adhesives which can adhere to a variety of substrates, including polyolefins.
Surprisingly, polyurethane-based structural adhesives have now been found which solve one or more problems encountered with known structural adhesives. In particular, polyurethane-based structural adhesives have been found which have better tack and cohesive properties, can adhere to a variety of substrates including polyolefins, and can be used up to high temperatures. It has now been found possible to provide such polyurethane-based structural adhesives by using a polyisocyanate and a polydiene polyol in the preparation of polyurethane.